Exchange symmetric pairs, stopping before the middle:
PROCEDURE reverse(i, n: INTEGER);
VAR t: INTEGER;
BEGIN
  t := 0;
  WHILE t < n DIV 2 DO
    swap(i+t, i+n-1-t);
    t := t + 1
  END
END reverse;
Writing for the pre-call array, the loop invariant says that the first and last t positions of the segment already satisfy , all positions between them retain their original values, and positions outside the segment are unchanged. Each swap fixes the next symmetric pair. The loop variant decreases; the two indices are distinct and in bounds whenever the body executes. Exactly swaps reverse the segment, with the odd-length middle element left in place and no accesses for .

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